Phase Relation and Equation of State of Iron‐Titanium Oxyhydroxides With α‐PbO2 Type Crystal Structure at Deep Mantle Conditions

Abstract Phase relation and equation of state of iron‐titanium oxyhydroxide with α‐PbO 2 ‐type crystal structure (hydrous α‐phase) was investigated at pressure from middle upper mantle to mantle transition zone. The experiments were performed using multi‐anvil apparatus with in situ synchrotron X‐ray diffraction. The metal‐diamond sample container was used to maintain a closed system with respect to water while allowing X‐ray transmittance. Starting materials were mixture of reagent grade goethite and anatase with Fe:Ti = 1:1 and 1:3. The hydrous α‐phase was synthesized at 11–14 GPa and 800–900°C. The X‐ray diffraction data were obtained over a wide range of pressures from 5 to 22 GPa and temperatures from room temperature to 930°C. Using pressure‐volume‐temperature data of the hydrous α‐phase with Fe:Ti = 1:1 collected at room temperature to 800°C and at 8.9–20.8 GPa, we determined the isothermal bulk modulus ( K T 0 = 183 (1) GPa) and the thermal expansivity ( α 0 = 3.29 (1) × 10 −5 K −1 ) at ambient condition. The stability field of the hydrous α‐phase and phase relation of FeOOH‐TiO 2 system at 900°C was well constrained. It was found that the hydrous α‐phase decomposes into baddeleyite‐type TiO 2 + ε‐FeOOH at pressure of approximately 20–21 GPa, and into ilmenite + rutile at 5–6 GPa. This stability field equivalent to depth of 180–600 km in the subduction zone. Our results suggest that the hydrous α‐phase is an important water reservoir in the middle upper mantle to mantle transition zone.